High power broadband waveguide window structure having septum to reduce reflection and ghost mode



3,183,459 RE HAVING ODE May 11, 1965 J. B. CAPUTO ETAL HIGH POWER BROADBAND WAVEGUIDE WINDOW STRUCTU SEPTUM TO REDUCE REFLECTION AND GHOST M Filed Oct. 4, 1965 FIG.1.

INVENTORS JAMES 5. CA 0 PUT THA 0051/5 I. SOKOL OWSK/ BY 9 la ATTORNEY I I l I l I g J' W I I u 52 5 4 5 6 5 8 6 0 4.2 4.4 4.6 4.8 5.0 4o FREQUENCY IN K.M.C.

United States Patent HIGH POWER BROADBAND WAVEGUEE WEN- DOW STRUQTURE HAVING SEPTUM T0 RE- DUCE REFLECTIGN AND GHOST MGDE James B. Caputo, Bayville, and Thaddeus I. Sokolowslri, Farmingdale, N.Y., assignors to Sperry Rand Col-para. tion, Great Neck, N.Y., a corporation of Deiaware Filed Oct. 4, 1963, Ser. No. 313,822 6 Ciaims. (Cl. 333-98) This invention relates to a waveguide window structure and more particularly to a broadband waveguide window structure that minimizes the presence of ghost modes in the dielectric material of the window, and therefore is well suited for operation at high power levels.

It has been shown in an article by M. P. Forrer and E. T. Jaynes entitled Resonant Modes in Waveguide Windows, appearing on pages 147-150 of IRE Transactions on Microwave Theory and Techniques, March 1960, that in the vicinity of certain Waveguide obstacles, such as planar dielectric windows, resonant electromagnetic field configurations can exist. These field configurations are known as ghost modes and are trapped in the immediate vicinity of the waveguide window, decaying exponentially on both sides of the window. The Q of the ghost mode can become exceedingly high, and these modes can store considerable energy at their resonant frequenc which may be different from the center frequency of the energy propagating in the dominant Waveguide mode. Because the trapped energy in the ghost mode is confined primarily within the dielectric material of a planar dielectric Window, the window is subjected to excessive heating, electrical breakdown, and possible physical rupture. The existence of these trapped ghost modes is a matter of major concern in dielectric waveguide window structures that are intended to operate at very high power levels because they limit the power level and bandwidth over which the window can operate successfully.

It therefore is an object of this invention to provide a high power, broadband waveguide window structure for effecting a strong hermetic barrier in a rectangular waveguide transmission line.

Another object of this invention is to provide a waveguide window structure in which the presence of ghost modes in the dielectric material of the window is minimized.

The present invention will be described by referring to the accompanying drawing wherein:

FIG. 1 is a perspective View, partially broken away, showing the waveguide window structure of the present invention.

FIG. 2 is a longitudinal sectional view of the window structure taken at section 22 of FIG. 1.

FIG. 3 is a graph showing the very broadband voltage standing wave ratio (V.S.W.R.) characteristic of the waveguide window structure of this invention.

In accordance with the illustrated embodiment of this invention, the window structure is comprised of first and second end sections of rectangular waveguide that are coupled by respective waveguide transition sections to a short length of circular waveguide. A planar disc of low loss dielectric material extends transversely across the central region of the circular section of waveguide and is sealed to the interior walls thereof to provide a hermetic window. A septum of conductive material that extends across the width of the circular waveguide section bisects the dielectric disc along a diametral plane and terminates at each end in the respective waveguide transition sections. The plane of the septum is oriented perpendicularly to the electric field vectors of the circular waveguide TE 3,183,459 Patented May 11, 1965 mode and effectively eliminates some and minimizes other ghost modes within the dielectric disc Window that have electric field lines orthogonal to the electric field lines of the dominant TE mode.

Referring now in detail to FIGS. 1 and 2, the structure is comprised of body members 11 and 12, the member 11 having a rectangular waveguide section 15 extending inwardly from its left end, the body member 12 having a rectangular waveguide section 16 extending inwardly from its right end. Rectangular Waveguide sections 15 and 16 have broad and narrow walls whose dimensions are proportioned in the ratio 2:1, in the usual manner, and propagate electromagnetic waves only in the dominant TE mode. Tapered waveguide sections 18 and 19 provide rectangular-to-circular-transition sections between rectangular waveguide sections 15 and 16 and the short section of circular waveguide section 20 that is located in the center region of the structure. A planar disc 22 of a low loss dielectric material extends across the entire cross sectional area of circular waveguide section 20 and is hermetically sealed within an oval-shaped rim member 25 that is secured between body members 11 and 12 by means of screws 27, for example. Iri order to reduce the possibility of breakdown between the rim 25 and the abutting faces of the flanged portions of members 11 and 12, it has been found to be helpful to make the abutting surface knurled. Circular waveguide 20 is of enlarged diameter in order to reduce the possibility of voltage reakdown in the region of'the waveguide window 22. A septum 3!) of high conductivity material bisects dielectric disc 22 through a diametrical plane, thus dividing disc 22 into two equal portions. Septum 30 extends completely across circular waveguide section 20 and is secured to the conductive walls thereof as by brazing. In the longitudinal direction, septum 30 extends to the beginning of the transition sections 18 and 19, and is tapered in these regions for impedance matching purposes.

It will be appreciated by those skilled in the art that waveguide section 20 need not be of true circular cross section but may be slightly modified in cross section to make allowance for the thickness of septum 30. This is a minor feature, so that the use of the word circular is intended to include any such minor variations.

If desired, a tubular aperture 32 may extend through the central region of septum 30 to permit the passage therethrough of a fluid coolant, thereby providing means for cooling dielectric disc 22. In practice, the region of septum 30 immediately surrounding aperture 32 may be comprised of a separate rectangular shaped bar 35 and the two halves of dielectric disc 22 may be sealed directly to the bar 35. In this event, the two end portions of septum 30 then may be joined to the rectangular bar 35 to provide the assembled septum.

Because the planar surfaces of thin conductive septum 3% are normal to the electric field vectors of the circular waveguide TE mode that propagates through circular Waveguide section 20, it has substantially no elfect on the propagation of that mode. It has been found by tests that the septum 3i effectively eliminates some and attenuates oth rs of the ghost modes within dielectric disc 22, these modes being the ones that have electrical field components lying in the plane of conductive septum 30. That is, it eliminates the electric fields of some ghost modes which are orthogonal to the electric field components of the dominant TE mode that propagates on both sides of septum 39 in circular Waveguide section 20. Conductive septum 39 also has an efiect to reduce the longitudinal extent of those ghost modes that have an electrical field component parallel to the electric field of the dominant TE mode. This attenuates the remaining ghost modes, thus reducing the amount of energy stored in these modes, thereby reducing the electrical gradient across the dielectric disc and reducing the possibility of electrical breakdown at the window. Also, dielectric heating and other undesirable effects are minimized.

Conductive septum 30 further provides the function of properly orienting the electromagnetic wave energy in the dominant TE mode in both halves of circular waveguide 20, thereby minimizing the possibility of reflections and breakdown in the waveguide transition sections of taining the dielectric disc windows. This mode has lon gitudinally extending components of electric field which have been found to contribute strongly to voltage breakdowns within the structure. Septum 30 of this device rapidly attenuates the TM mode and permits the structure to operate satisfactorily at very high power levels. Another advantage of eliminating longitudinally extending components of electric field in the region of dielectric disc 22 is that the bombardment of the dielectric disc 22 by free electrons is reduced. This bombardment, known as multipactor, results from free electrons being accelerated along the longitudinally extending electric field lines that wouldintersect the plane surface of the dielectric disc.

Septum 30 also increases the mechanical strength ofthe relatively thin dielectric disc 22. If desired, additional septa, parallel to septum 30 but vertically displaced therefrom, may extend through dielectric disc 22. This would aid in further reducing the presence of ghost modes since the possibility of their existence is, in part, a function of the height of the dielectric material of the window.

In one device constructed in the manner illustrated in FIG. 1 and intended for operation in the C-band of the" microwave frequency spectrum, the device exhibited a maximum voltage standingwave ratio (V.S.W.R.) of 1.10 over the frequency ranges of 4,000 to 6,000 megacycles. A plot of this response is illustrated in FIG. 3. This exceedingly low V.S.W.R. extends substantially throughout the entire C-band and represents a bandwidth that is unusually wide in the art of microwave windows, as will be appreciated by those skilled in this art. Because a high V.S.W.R. contributes to voltage breakdown, it is evident that the window structure of this invention is well suited for operation at high power levels.

As an alternative, the surfaces of septum 30 may be coated with a material that is dissipative to electromagnetic waves, rather than being conductive. The overall operation of the device will be substantially the same except possibly the device may show a slightly higher insertion loss because of the presence of the dissipative material.

In the device constructed in the manner illustrated in FIGS. 1 and 2, the electrical length of the circular waveguide section 20 was approximately one-quarter wavelength at the center frequency of operation, but we have found that with the presence of conductive septum 30, this electric length is not critical.

In practice we have used for the dielectric disc 22 a ceramic material known as Coors AD99, obtained from Coors Porcelain Company, Golden, Colorado.

A waveguide window structure constructed substantially as illustrated in FIGS. 1 and 2 had the following approximate dimensions:

Inches Length of rectangular waveguide sections 15, 16 .75 Length of transition sections 18, 19 1.00 Length of circular waveguide section 20 .80

4 Length of septum 30 2.8 Thickness of septum 30 .34 Length of taper on septum 30 1.00 Thickness of dielectric disc window 22 .065 Radius of each half of dielectric disc 22 1.00

From the above description it may be seen that the addition of the conductive septum 30 to the window structure comprised of a planar dielectric disc positioned within a circular waveguide section has effectively eliminated many of the troublesome ghost modes that otherwise would have existed within the dielectric disc 22, thereby increasing the high power capabilities of the window structure.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

1. A waveguide window structure for establishing a broadband, strong hermetic seal across a section of circular waveguide, said window structure comprising,

a disc of dielectric material disposed transversely across said circular waveguide section and sealed along its periphery to said Waveguide section,

said disc being permeable to electromagnetic waves and impervious to fluids,

a longitudinally extending septum extending through said dielectric disc along a diametral plane to divided said disc into two substantially equal portions, and

means for coupling electromagnetic waves in the dominant TE mode into said circular waveguide with the electric field of said waves normal to the broad surface of said septum.

2. The combination claimed in claim 1 wherein the surface of said septum is made of a conductive material. 3. The combination claimed in claim 1 wherein said septum is provided with a surface of material that is dissipative to electromagnetic waves.

4. The combination claimed in claim 1 and further including means in said septum for passing a fluid coolant through said septum, thereby to cool said dielectric disc. 5. A high power waveguide window structure comprising,

first and second end waveguide sections of rectangular cross-section adapted to propagate in the dominant TE mode electromagnetic waves within a given frequency range,

a section of circular waveguide disposed intermediate said rectangular waveguide sections,

waveguide means for coupling electromagnetic Waves from the TE mode in said rectangular waveguides to the TE mode in said circular waveguide,

a disc of low loss dielectric material disposed transversely across the central region of said circular waveguide and sealed to the interior surface of said waveguide to form a hermetic seal, and

a longitudinally extending septum extending through said dielectric disc along a diametral plane normal to the electric field vectors of waves in said TE mode,

said septum being adapted to substantially eliminate from said disc trapped ghost modes that have electric field components parallel to the surface of the septum.

6. A high power waveguide window structure comprising,

first and second axially-spaced sections of rectangular waveguide having broad and narrow walls and adapted to propagate electromagnetic waves in the dominant TE mode,

first and second sections of rectangular-to-circular waveguide transition sections respectively coupled to the adjacent ends of said rectangular Waveguide sections,

a section of circular waveguide section coupled between said transition sections and adapted to propagate electromagnetic waves in th circular waveguide TE mode,

said circular waveguide section having a diameter larger than said transition sections and having an electrical length approximately equal to a quarter Wavelength at the center frequency of electromagnetic waves propagating through said device,

a disc of low loss dielectric material extending across the transverse central region of said circular waveguide section and secured to the inner walls thereof to provide a hermetic window in said waveguide, and

a conductive septum extending longitudinall through said dielectric disc along a diametral plane that is normal to the electric field polarization of the dominant 'IE mode propagating in said circular Waveguide section,

said septum extending transversely to the walls of said circular Waveguide and longitudinally to said rectangular Waveguide sections, thereby to eliminate within said dielectric disc ghost modes having longitudinal components of electric field in the plane of said septum.

References Cited by the Examiner UNITED STATES PATENTS 2,129,669 9/38 Bowen 333--21 2,958,834 11/60 Symons 33398 3,110,960 11/63 Churchill 333-98 HERMAN KARL SAALBACH, Primary Examiner. 

1. A WAVEGUIDE WINDOW STRUCTURE FOR ESTABLISHING A BROADBAND, STRONG HERMETIC SEAL ACROSS A SECTION OF CIRCULAR WAVEGUIDE, SAID WINDOW STRUCTURE COMPRISING, A DISC OF DIELECTRIC MATERIAL DISPOSED TRANSVERSELY ACROSS SAID CIRCULAR WAVEGUIDE SECTION AND SEALED ALONG ITS PERIPHERY TO SAID WAVEGUIDE SECTION, AND DISC BEING PERMEABLE TO ELECTROMAGNETIC WAVES AND IMPERVIOUS TO FLUIDS, A LONGITUDINALLY EXTENDING SEPTUM EXTENDING THROUGH SAID DIELECTRIC DISC ALONG A DIAMETRAL PLANE TO DIVIDED SAID DISC INTO TWO SUBSTANTIALLY EQUAL PORTIONS, AND MEANS FOR COUPLING ELECTROMAGNETIC WAVES IN THE DOMINANT TE11 MODE INTO SAID CIRCULAR WAVEGUIDE WITH THE ELECTRIC FIELD OF SAID WAVES NORMAL TO THE BROAD SURFACE OF SAID SEPTUM. 